How Al-Hasan ibn 'Ali died
6-4-2016 | IslamWeb
Question:
How did Al-Hasan ibn Ali, may Allaah be pleased with him, die? Was he martyred in a war or did he die a natural death?
Answer:
All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.
It has been reported that Al-Hasan 
died of poisoning. The scholars held different views regarding who poisoned him; some said that it was done by Muʻaawiyah, others said that it was Yazeed ibn Muʻaawiyah, and others said that it was his wife. However, none of these accounts are authentically proven true.
Ibn Taymiyyah 
said in his book Minhaaj As-Sunnah:
“As for his claim (i.e. the Raafidhi Ibn Al-Mutahhar Al-Hilli) that Muʻaawiyah poisoned Al-Hasan, this is what some people have said, but it is not proven with reliable evidence or reasonable admission, nor through any report that can be trusted. Therefore, this is a matter that cannot be known, and any statement regarding it would be a statement without knowledge. In our time, we hear people saying that so and so was poisoned, referring to kings or others. People disagree about that and even disagree about the place where he died and the fortress in which he died. You will find each of them relating a different account from what the others related. It was said that Al-Hasan was poisoned, and this can be easily known because if a person died of poisoning, this is easily discovered. However, it was said that it was his wife who poisoned him. There is no doubt that he died in Madeenah while Muʻaawiyah was in Shaam (Syria). Someone might assume that Muʻaawiyah sent someone to her, ordering her to poison him. It may also be argued that she poisoned him for any other reason for which a woman may poison her husband; it has been reported that Al-Hasan divorced many of his wives and did not remain with a wife for a long period of time. It is also said that her father Al-Ashʻath ibn Qays ordered her to poison him; Al-Ashʻath ibn Qays was accused of secretly rebelling against ʻAli and his son Al-Hasan. If it is claimed that Muʻaawiyah ordered her father Al-Ashʻath ibn Qays to command her to do so, then that is mere assumption. The Prophet, sallallaahu ʻalayhi wa sallam, said, ‘Beware of (negative) assumption, for (negative) assumption amounts to the worst form of lying.’ [Al-Bukhari]
In general, by the consensus of the scholars, such unsupported claims are of no consequence in the sharee'ah. They entail neither praise nor dispraise of anyone, and Allaah knows best. Moreover, Al-Ash'ath ibn Qays died in the year 40 or 41 AH, and thus was not mentioned in the account of the reconciliation between Mu'aawiyah and Al-Hasan ibn 'Ali in the year 41 AH. Al-Ash'ath was the father-in-law of Al-Hasan ibn 'Ali. Had he witnessed the reconciliation, his name would have been mentioned. And if he had died ten years before Al-Hasan's death, then how could he have been the one who ordered his daughter to poison Al-Hasan? Allaah Almighty knows best the truth of what happened, and He will judge between His servants in what they disputed about. If any of that did occur, then it was part of their fighting among each other, as has been mentioned earlier. The fighting of the Muslims among each other is subject to interpretation, and their verbal abuse of each other is subject to interpretation, and their accusing each other of infidelity is subject to interpretation. It is a serious subject, and the one who has no knowledge of it must avoid it, otherwise, he might go astray.” [End of quote]
His wife in reference is Jaʻdah bint Al-Ashʻath ibn Qays.
Among those who criticized the claim that Muʻaawiyah poisoned Al-Hassan was Abu Bakr ibn Al-ʻArabi in his book Al-ʻAwaasim min Al-Qawaasim.
It was also claimed that it was Yazeed ibn Muʻaawiyah who poisoned him, but this is not substantiated with evidence. Ibn Katheer wrote, “Some of them have narrated that Yazeed ibn Muʻaawiyah wrote to Jaʻdah to poison Al-Hasan and that he would marry her afterwards. So she carried it out, and after Al-Hasan passed away, she sent him a message and he replied, ‘By Allaah, we did not approve of you for Al-Hasan; would we approve of you for ourselves?’ In my opinion, this is not authentic; and it is inauthentic regarding his father, Muʻaawiyah, with greater reason.” [Al-Bidaayah wa An-Nihaayah]
Ibn Katheer also wrote, “... When Al-Hasan was dying, the physician said, ‘This man’s intestines have been torn by poison.’ His brother, Al-Husayn, asked him, ‘Who did this to you?’ Al-Hasan said, ‘Why, Brother (do you ask)?’ Al-Husayn replied:,‘By Allaah, I will kill him before I bury you, unless I am not able to reach him or he is in a so far land that I cannot go to him.’ Al-Hasan said, ‘O my brother, this life is but fleeting nights; leave him until we meet before Allaah,’ and he refused to name him.”
May Allaah confer His mercy upon Al-Hasan, and .
Allaah knows best.